Active phased array antennas are composed of many radiating elements, each with phase and amplitude control. Beams are formed by weighing the amplitude and shifting the phase of the signal emitted from each radiating element, to provide constructive/destructive interference so as to steer the beams in the desired direction. Planar phased array antennas can electronically steer the beam in azimuth and the elevation plane and provide faster beam steering rates than a mechanical steering system.
A phased array antenna must be calibrated in the factory before being deployed in the field in order to ensure that the radiation pattern of the antenna meets the antenna performance specification. The calibration is typically performed in a near-field antenna range; during this process a sampling probe is positioned in front each radiating element, with that element in either transmit or receive mode and the remaining array elements terminated in matched loads. The amplitude and phase of each radiating element is accurately measured through each T/R module amplitude and phase state. This data is used to develop correction factors that minimize the element-to-element random errors. The desired radiation pattern is then achieved by adjusting the T/R module amplitudes and phases as indicated by the corrections factors.
Solid-state radars for weather application require the phased array antennas to be deployed for a long period of time. The performance of the antennas may deteriorate over time as a result of changes in the solid-state devices. In addition, failed T/R modules must be replaced in the field. As a result, the T/R modules must be re-calibrated to correct the component drift or the module replacement. In order to avoid the radars being taken out of service for a long time, a self-calibration is required.
Several techniques of self-calibration or auto-calibration for active phased array antennas have been proposed and implemented. These have an internal calibration source and use mutual coupling measurements among array elements to determine the element-to-element errors. Some such calibration techniques use the inherent array mutual coupling to transmit and receive signals between pairs of adjacent elements in the array, while all other elements are turned off and terminated in matched loads.
Another calibration technique employs a small number of dedicated passive calibration elements to calibrate the antenna. The array is split into several blocks, each having a single passive calibration element near its center. The calibration is achieved by sequentially measuring the mutual coupling between each passive element and a selected group of active array elements belonging to adjacent blocks. However, this technique is not suitable for small array antennas, because a small number of dedicated passive elements for calibration would degrade the sidelobe level and antenna gain.